The closed-loop precoding scheme in Long Term Evolution Advanced (LTE-A) is extended to support configuration with up to 8 transmit antennas in the downlink using a wireless transmit/receive unit (WTRU)-specific reference signal to improve data coverage. Since the WTRU-specific demodulation reference signal (DM RS) is precoded in the same way as the data, a non-codebook based precoding may be applied.
The orthogonality among the DM RS ports in the LTE Release 10 is achieved using a combination of code division multiplexing (CDM) and frequency division multiplexing (FDM). FIG. 17 shows a conventional WTRU-specific reference signal pattern for antenna ports 7, 8, 9, and 10 for normal cyclic prefix. Total 12 resource elements (REs) are used per layer for each subframe. Two CDM groups occupy different subcarriers in a frequency domain. In LTE Release 10, antenna ports 7, 8, 11, and 13 belong to CDM group 1, and antenna ports 9, 10, 12, and 14 belong to CDM group 2. For rank-1 to rank-4, a length-2 orthogonal cover code (OCC) is applied on two adjacent REs in each slot while for rank-5 to rank-8, a length-4 OCC is applied on two groups of consecutive REs in the subframe.
A WTRU receives information regarding the assigned DM RS antenna port(s), the scrambling identity, and the number of layers through downlink control information, (e.g., a 3-bit field in DCI format 2C as specified in Table 1). nSCID is the scrambling identity for antenna ports 7 and 8.
TABLE 1One Codeword:Two Codewords:Codeword 0 enabled,Codeword 0 enabled,Codeword 1 disabledCodeword 1 enabledValueMessageValueMessage
In LTE Release 10, for multi-user multiple-input multiple-output (MU-MIMO) operation, up to four layers can be simultaneously scheduled on the same frequency-time resources. This is accomplished through multiplexing of layers on two orthogonal DM RS antenna ports (antenna ports 7 and 8) with two scrambling sequences, nSCID={0,1} (the shaded rows in Table 1).